Port flat nesting to Bricks 8.x

In order to support flat nesting in Bricks 8.x we need to port some of Treeable (submodule of Tree) functionality:

1) Ability to edit a tree of field items with Drupal's table drag (or probably more specific Entity Reference field widget).

2) Ability to store a tree of field items instead of flat list (in Bricks 7.x this is implemented via Entity Reference Behaviors).